Clockless delay adaptation loop for random data

ABSTRACT

An apparatus includes a clockless delay adaptation loop configured to adapt to random data. The apparatus also includes a circuit coupled to the clockless delay adaptation loop. The clockless delay adaptation loop includes a cascaded delay line and an autocorrelation control circuit coupled to the cascaded delay line, wherein an output of the autocorrelation control circuit is used to generate a control signal for the cascaded delay line.

BACKGROUND

The proliferation of networked electronic devices continues along withrelated efforts to increase signaling speed while maintaining signalintegrity. The core of a communication link consists of a transmitterthat generates the signal, a channel that carries the signal and areceiver that accepts the signal and processes it correctly. Signals canbe carried by channels as electrical signals, optical pulses, orelectromagnetic signals. For accurate signaling between networkeddevices or components, the integrity of the signal needs to bemaintained. From a physical layer standpoint, this means that if a “1”is sent from the transmitter and down the channel, be it copper, opticalfiber or air, the receiver should also determine that the signal is a“1”. This is a fundamental challenge for data transmission system andcircuit designers due to the various factors affecting signal integrity.

Signal integrity in data transmission systems is affected by a number offactors. Apart from random noise, atmospheric and man-made noise, thereare deterministic factors that contribute to deteriorating signalintegrity. Copper-based signal transmission is affected by the limitedbandwidth of copper channels and crosstalk from adjacent channels.Optical signals are affected by frequency-independent loss of opticalpower as they travel down the fiber as well as dispersion-causingmechanisms depending on the type of fiber. Bandwidth limitations incopper and pulse dispersion in optical fibers result ininter-symbol-interference (ISI) at the receiver. Typically, ISIdeteriorates as the data rate and the length of the channel increases.ISI is the primary factor limiting transmission distances overcopper-based transmission channels and optical fiber at high data-rates(e.g., 10-Gb/sec). Efforts to compensate for ISI and otherwise maintainsignal integrity has resulted in various transmitter-side andreceiver-side technologies, including equalization.

Equalization is a process of conditioning the electrical signal, eitherat the transmitter or the receiver to compensate for channel-induced ISIand improve signal integrity. Linear and non-linear equalizationtechniques have been explored in the literature. One non-linearequalization technique is referred to as decision-feedback equalization(DFE). In DFE, a delay element is introduced into the receiver circuitryand equalization values are combined with an input data stream.

In many applications (e.g., re-drivers), a clock is not available andthus a clockless delay line (to delay the random data) is needed. Thedelay needs to be adapted automatically to 1 unit interval (UI) ormultiples of 1 UI for a range of permissible data rates. Delayadaptation for different data rates is not trivial and issues such ascost, power efficiency, and footprint size need to be considered.Efforts to improve DFE design and account for different data rates areongoing.

SUMMARY

In accordance with at least one example of the disclosure, an apparatuscomprises a clockless delay adaptation loop configured to adapt torandom data. The apparatus also comprises a circuit coupled to theclockless delay adaptation loop. The clockless delay adaptation loopcomprises a delay line with a data input node, a data output node, and acontrol input node. The clockless delay adaptation loop also comprisesan autocorrelation control circuit coupled to the cascaded delay line,wherein an output of the autocorrelation control circuit is used togenerate a control signal for the cascaded delay line.

In accordance with at least one example of the disclosure, a clocklessdecision-feedback equalizer (DFE) comprises a summation circuitconfigured to provide an output signal by combining an input signal anda feedback signal. A DFE feedback path configured to receive the outputsignal and provide the feedback signal, wherein the DFE feedback pathincludes a delay adaptation loop configured to adapt to random data. Thedelay adaption loop comprises a voltage-controlled delay line (VCDL)with a data input node, a data output node, and a control input node.The delay adaptation loop also comprises an autocorrelator coupled tothe data input node and the data output node, wherein an output of theautocorrelator is used to generate a control signal input to the controlinput node. The delay adaptation loop also comprises a DFE tap circuitin series with the VCDL.

In accordance with at least one example of the disclosure, a delayadaptation loop comprises a first delay line and a second delay line.The delay adaption loop also comprises an autocorrelation controlcircuit coupled to the first delay line and the second delay line. Theautocorrelation control circuit provides a control signal to each of thefirst and second delay lines based on an autocorrelation functionapplied to input data and output data of at least one of the first andsecond delay lines.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of various examples, reference will now bemade to the accompanying drawings in which:

FIG. 1 is a diagram showing a communication system in accordance withsome examples;

FIG. 2 is a diagram showing a clockless decision feedback equalization(DFE) loop in accordance with some examples;

FIG. 3A is a graph showing autocorrelation using an XNOR gate withoutinter-symbol interference (ISI);

FIG. 3B is a graph showing autocorrelation using an XNOR gate with ISI;

FIG. 4 is a diagram of a delay adaptation loop in accordance with someexamples;

FIG. 5 is a graph showing delay and control voltage (V_(ctrl)) curves inaccordance with some examples;

FIGS. 6A and 6B are graphs showing adaptive delay settling in accordancewith different examples;

FIGS. 7A and 7B are additional graphs showing adaptive delay settling inaccordance with different examples;

FIGS. 8-10 are diagrams other delay adaptation loops in accordance withsome examples;

FIG. 11 is a schematic diagram of a voltage-controlled delay line (VCDL)in accordance with some examples;

FIG. 12 is a schematic diagram of a current-mode logic (CML) XNOR gatein accordance with some examples; and

FIG. 13 is a schematic diagram of a transconductance (G_(m)) stage inaccordance with some examples.

DETAILED DESCRIPTION

Disclosed herein are delay adaptation loop topologies that employ anautocorrelation function. The autocorrelation function of a random datastream decreases monotonically as the delay shift is increased from zerodelay, and goes to zero when the time shift reaches and exceeds 1 unitinterval (UI). In the presence of inter-symbol interference (ISI), theautocorrelation function may have a non-zero tail that exceeds a timeshift of 1 UI. So instead of locking to 1 UI, a more robust optioninvolves locking to 0.5 UI by forcing the autocorrelation functiontowards 0.5 (instead of 0). As desired, one or more 0.5-UI delayelements are cascaded to generate the 1-UI delay. As described herein,example delay elements include voltage-controlled delay lines (VCDLs)and a decision-feedback equalization (DFE) tap circuit, where VCDLs havean adjustable delay and the DFE tap circuit has a fixed delay.

In some examples, a delay adaptation loop includes a VCDL with a datainput node, a data output node, and a control input node. The delayadaptation loop also includes an autocorrelator coupled to the datainput node and the data output node of the VCDL. The delay adaptationloop also includes a transconductance (G_(m)) stage coupled to theautocorrelator, wherein the transconductance stage is configured toconvert a voltage corresponding to a portion (e.g., 0.5) of anautocorrelator output into a current. The delay adaptation loop alsoincludes a capacitor with a first (e.g., top) plate coupled to an outputof the transconductance stage and with a second (e.g., bottom) platecoupled to a ground node. The output of the transconductance stage isalso coupled to the control input node of the VCDL to lock the VCDL to avalue of less than 1 UI. As desired, additional VCDLs and/or a DFE tapcircuit are coupled to the output of the transconductance stage. Inother examples, a delay adaptation loop includes multiples VCDLs and anautocorrelator for each VCDL. In such case, the autocorrelator outputsare used separately (e.g., separate transconductance stages are used) orare combined (e.g., a single transconductance stage are used).

In some examples, the delay adaptation loop is employed in a clocklessdecision-feedback equalizer (DFE). Also, in different examples, theclockless DFE is part of a linear repeater or redriver circuit. With thedisclosed delay adaptation loop, a delay locked loop is able to adapt to1 UI delay (or some multiple of 0.5 UI) for random data without the needfor any clock in the system. To provide a better understanding, variousdelay adaptation loop options, system options, autocorrelation options,and related circuits are described using the figures as follows.

FIG. 1 is a diagram showing a communication system 100 in accordancewith some examples. As shown, the system 100 includes a transmitter 104,a channel 106, a linear repeater or redriver 110, a channel 118, and areceiver 120. In some examples, the communication system includes ACcouple capacitors (C_(AC1) and C_(AC2)) before and after the linearrepeater or redriver 110. In FIG. 1, source data 102 is output from thetransmitter 104 and is represented by transmitted signal eye diagram105. After conveyance via the channel 106, the source data 102 is nolonger distinguishable as represented by the received signal eye diagram107. The operation of the linear repeater or redriver 110 is to recoverthe source data 102 transmitted by the transmitter 104 and to repeat thesame signal using a driver 116, where the data transmitted by the driver116 is represented by the transmitted signal eye diagram 117. Therepeated data is transmitted by the driver 116 to the channel 118. Inthe example of FIG. 1, the linear repeater or redriver 110 recovers thedata using a continuous time linear equalizer (CTLE) 112 and a clocklessDFE loop 114. In the example of FIG. 1, the DFE loop 114 includes adelay adaptation loop 115 with an autocorrelator and a cascaded delayline as described herein. In some examples, the cascaded delay lineincludes at least one VCDL and a DFE tap circuit. The equalizationoperations of the CTLE 112 and the clockless DFE 114 facilitate recoveryof the source data 102.

More specifically, the channel 106 represents one or more physicaltransmission mediums (e.g., wires, air, optical fiber, etc.) used toconvey signals between the transmitter 104 and the linear repeater orredriver 110. In operation, the transmitter 104 receives the source data102 and conveys a signal representing the source data 102 to the channel106. The linear repeater or redriver 110 operates to receive signalsfrom the channel 106, to recover the source data 102, and to pass therecovered data linearly to another communication link.

Recovering the source data 102 at the linear repeater or redriver 110involves various operations, including those performed by the clocklessDFE loop 114. The operations of the clockless DFE loop 114 includebriefly delaying and modifying receiver-side signals, resulting in“equalized” signals, which are then used by the driver 116 to generate anew signal that matches the source data 102. With the clockless DFE loop114, higher-speed signaling due to a reduction in intersymbolinterference (ISI) is possible.

FIG. 2 is a diagram showing a clockless DFE loop 200 in accordance withsome examples. As shown, the clockless DFE loop 200 includes a summationcircuit 208 configured to provide an output signal 214 by combining aninput signal 202 and a feedback signal 216. The clockless DFE loop 200also comprises a delay adaptation loop 204 configured to adapt to randomdata. In the example of FIG. 2, the delay adaptation loop 204 receivesthe output signal 214 as an input and provides the feedback signal 216as an output. As shown, the delay adaptation loop 204 includes anautocorrelation control circuit 206 and a cascaded delay line 206, wherethe autocorrelation control circuit 206 receives input/output data fromthe cascaded delay line 206 and provides at least one control signal(e.g., V_(ctrl), V_(ctrl1), V_(ctrl2)) to the cascaded delay line 206.In different examples, the autocorrelation control circuit 206 varieswith regard to the number of autocorrelators and/or other componentsused to generate at least one control signal (e.g., V_(ctrl), V_(ctrl1),V_(ctrl2)). In operation, the autocorrelation control circuit 206correlates input/output (I/O) data related to at least one of the delaylines of a cascaded delay line 205. The correlation results are used togenerate at least one control signal (e.g., V_(ctrl), V_(ctrl1),V_(ctrl2)), which is provided to the delay lines of the cascaded delayline 205. In some examples, the cascaded delay line 205 includes asingle VCDL and a DFE tap circuit (a delay line with a fixed delay witha programmable output swing) configured to generate the feedback signalby applying signed DFE tap weights to a delayed version of the outputsignal 214. In other examples, the cascaded delay line 205 includes aseries of VCDLs as well as a DFE tap circuit configured to apply signedDFE tap weights to a delayed version of the output signal 214.

FIG. 3A is a graph 300 showing autocorrelation using an XNOR gatewithout inter-symbol interference (ISI). The autocorrelation function ofa real signal f(t) is given by:

${R_{f}(\tau)} = {\lim\limits_{T\rightarrow\infty}{\frac{1}{2T}{\int_{- T}^{T}{{f(t)}{f\left( {t - \tau} \right)}d{t.}}}}}$For a random non-return-to-zero (NRZ) bit stream, the autocorrelationfunction is a triangular function as shown on the right that goes to(and stays at) zero at (beyond) 1 UI. For a random NRZ bit stream, anXNOR gate (output: ±1 V) is usable as an autocorrelator.

FIG. 3B is a graph 310 showing autocorrelation using an XNOR gate withISI. In the presence of ISI, the tail spreads as shown in graph 310, butthe 0.5 UI point 312 does not shift much. Hence, one proposed strategyinvolves comparing the average XNOR gate output (output: ±1 V) to 0.5 Vand locking the delay to 0.5 UI.

FIG. 4 is a diagram of a delay adaptation loop 400 in accordance withsome examples. The delay adaptation loop 400 is an example of the delayadaption loop 204 in FIG. 2, which is part of a clockless DFE. In otherexamples, the delay adaption loop 400 is used in a clockless feedforwardequalizer (FFE), a loss of lock detector, and other such applicationsacross data rates. As shown, the delay adaptation loop 400 includes aVCDL 402 in series with a VCDL 404, which form a cascaded delay line(e.g., the cascaded delay line 205 in FIG. 2). The delay adaption loop400 also includes an autocorrelator 406 coupled to the input and outputof the VCDL 402. In the example of FIG. 4, the autocorrelator 406 is aXNOR gate, which correlates the input and output of the VCDL 402. Insome examples, the average autocorrelator output is set to 0.5 using atransconductance stage 408 that subtracts 0.5V from the output of theautocorrelator 406. The output of the transconductance stage 408 is usedto charge a capacitor (C1), which determines V_(ctrl). In the example ofFIG. 4, V_(ctrl) is provided as a control signal to the VCDL 402 and404, which locks the delay for each of the VCDLs 402 and 404 at 0.5 UI(resulting in a total delay of 1 UI for the delay adaptation loop ofFIG. 4). In FIG. 4, the autocorrelator 406, the transconductance stage408, and C1 correspond to an autocorrelation control circuit (e.g., theautocorrelation control circuit 206 of FIG. 2).

FIG. 5 is a graph 500 showing delay and V_(ctrl) curves in accordancewith some examples. In graph 500, an autocorrelation function (e.g., asin FIG. 2 or 4) is used to generate the delay and V_(ctrl) curves. Inthe example of FIG. 5, the delay curve settles or locks to 2×0.5 UI(˜62.5 ps) as V_(ctrl) settles, resulting in a reasonably closed inputeye (not shown).

FIGS. 6A, 6B, 7A, and 7B are graphs showing adaptive delay settling inaccordance with different examples. In graph 600 of FIG. 6A, apseudorandom binary sequence (PRBS7) is used to generates a delay curvebased on an autocorrelation function, where 10-in FR4 at 16 Gb/s isused. In graph 600, the delay curve settles such that the peak-to-peakvariance after about 18 us is 40 fs. In graph 610 of FIG. 6B, apseudorandom binary sequence (PRBS15) is used to generates a delay curvebased on an autocorrelation function, where 10-in FR4 at 16 Gb/s isused. In graph 610, the delay curve settles such that the peak-to-peakvariance after about 18 us is 800 fs (longer PRBS run results in moredelay jitter/variation. Delay variation in steady state is also afunction of the bandwidth (BW) of the delay adaptation loop.

In graph 700 of FIG. 7A, delay curves are represented for initial delaysgreater than or less than 1 UI, where an autocorrelation function isused to adjust the delay as needed. As represented in graph 700, anautocorrelation function is effective regardless of whether an initialdelay is greater than or less than 1 UI (autocorrelation supportsbidirectional locking). For the graph 700, PRBS7 and 10-in FR4 at 16Gb/s is used. In graph 710 of FIG. 7B, different delay curvescorresponding to different data rates are represented, whereautocorrelation, 10-in FR4 channel and PRBS 7 are used to generate thedelay curves. As represented in graph 710, if the delay line has enoughrange, the proposed autocorrelation function automatically adapts thedelay across data rates.

If delay mismatch of two VCDLs is a concern, independent autocorrelationloops around each VCDL is possible. FIG. 8 is a diagram of a delayadaptation loop 800 with independent autocorrelation loops in accordancewith some examples. The delay adaptation loop 800 is an example of thedelay adaption loop 204 in FIG. 2, which is part of a clockless DFE. Inother examples, the delay adaption loop 800 is used in a clockless FFE,a loss of lock detector, and other such applications across data rates.

As shown, the delay adaptation loop 800 includes a VCDL 802 in serieswith a VCDL 804, which form a cascaded delay line (e.g., the cascadeddelay line 205 in FIG. 2). The delay adaption loop 800 also includes anautocorrelator 806 coupled to the input and output of the VCDL 802.Also, an autocorrelator 816 is coupled to the input and output of theVCDL 804. In the example of FIG. 8, the autocorrelators 806 and 816 areXNOR gates, where the autocorrelator 806 correlates the input and outputof the VCDL 802, and where the autocorrelator 816 correlates the inputand output of the VCDL 804. In some examples, the average autocorrelatoroutput for the autocorrelator 806 is set to 0.5 using a transconductancestage 808 that subtracts 0.5V from the output of the autocorrelator 806.The output of the transconductance stage 808 is used to charge acapacitor (C2), which determines a control signal (V_(ctrl1)) for theVCDL 802. Also, the average autocorrelator output for the autocorrelator816 is set to 0.5 using a transconductance stage 818 that subtracts 0.5Vfrom the output of the autocorrelator 816. The output of thetransconductance stage 808 is used to charge a capacitor (C3), whichdetermines a control signal (V_(ctrl2)) for the VCDL 804. With V_(ctrl1)and V_(ctrl2), the delays for each of the VCDLs 802 and 804 are lockedat 0.5 UI (resulting in a total delay of 1 UI for the delay adaptationloop 800 of FIG. 8). In FIG. 8, the autocorrelators 806 and 816, thetransconductance stages 808 and 818, C2, and C3 correspond to anautocorrelation control circuit (e.g., the autocorrelation controlcircuit 206 of FIG. 2).

As another option, two autocorrelators are used for the two 0.5 UIdelays, where the outputs of the autocorrelators are averaged beforegoing through a Gm stage. FIG. 9 is a diagram of a delay adaptation loop900 with averaged autocorrelation loops in accordance with someexamples. The delay adaptation loop 900 is an example of the delayadaption loop 204 in FIG. 2, which is part of a clockless DFE. In otherexamples, the delay adaption loop 900 is used in a clockless FFE, a lossof lock detector, and other such applications across data rates.

As shown, the delay adaptation loop 900 includes a VCDL 902 in serieswith a VCDL 904, which form a cascaded delay line (e.g., the cascadeddelay line 205 in FIG. 2). The delay adaption loop 900 also includes anautocorrelator 906 coupled to the input and output of the VCDL 902.Also, an autocorrelator 916 is coupled to the input and output of theVCDL 904. In the example of FIG. 9, the autocorrelators 906 and 916 areXNOR gates, where the autocorrelator 906 correlates the input and outputof the VCDL 902, and where the autocorrelator 916 correlates the inputand output of the VCDL 904. In the example of FIG. 9, the outputs of theautocorrelators 906 and 916 are combined (averaged) using resistors R1and R2, where the average autocorrelator outputs for the autocorrelators906 and 916 is set to 0.5 using a transconductance stage 908 thatsubtracts 0.5V from the averaged outputs of the autocorrelators 906 and916. The output of the transconductance stage 908 is used to charge acapacitor (C4), which determines a control signal (V_(ctrl)) for theVCDLs 902 and 904. In FIG. 9, the autocorrelators 906 and 916, R1, R2,the transconductance stage 908, and C4 correspond to an autocorrelationcontrol circuit (e.g., the autocorrelation control circuit 206 of FIG.2). With the delay adaptation loop 900, the loop will lock to a totaldelay of 1 UI even if each delay line is slightly off from 0.5 UI due tomismatch (e.g., process, temperature, or supply voltage mismatch).

As another option, is some examples, filtering is performed using adigital accumulator/integrator that counts the pulses coming out of theXNOR gate. In such case, the loop is be closed with either adiscretely-tuned delay line or a DAC. FIG. 10 is a diagram of a delayadaptation loop 1000 with autocorrelation and a digital state machine inaccordance with some examples. The delay adaptation loop 1000 is anexample of the delay adaption loop 204 in FIG. 2, which is part of aclockless DFE. In other examples, the delay adaption loop 1000 is usedin a clockless FFE, a loss of lock detector, and other such applicationsacross data rates.

As shown, the delay adaptation loop 1000 includes a VCDL 1002 in serieswith a VCDL 1004, which form a cascaded delay line (e.g., the cascadeddelay line 205 in FIG. 2). The delay adaption loop 1000 also includes anautocorrelator 1006 coupled to the input and output of the VCDL 1002. Inthe example of FIG. 10, the autocorrelator 1006 is a XNOR gate, wherethe autocorrelator 1006 correlates the input and output of the VCDL1002. As shown, the output of the autocorrelator 1006 is provided to adigital state machine 1008. In the example of FIG. 10, the digital statemachine includes a counter 1010 and an accumulator+comparator block1012. The output of the digital state machine 1008 is fed into a DAC1014, where the output of the DAC 1014 is a control signal (V_(ctrl))for the VCDLs 1002 and 1004. In FIG. 10, the autocorrelator 1006, thedigital state matching 1008 and the DAC 1014 correspond to anautocorrelation control circuit (e.g., the autocorrelation controlcircuit 206 of FIG. 2). With the delay adaptation loop 1000, the loopwill lock to a total delay of 1 UI.

FIG. 11 is a schematic diagram of a VCDL 1100 (e.g., the VCDLs in FIGS.2, 4, and 8-10) in accordance with some examples. As shown, the VCDL1100 includes a positive input node (V_(inp)) and a negative input node(V_(inn)). The VCDL 1100 also includes resistors (R3-R6) and varactors(C5-C8) in the arrangement shown. Also represented in FIG. 11 is apositive output node (V_(outp)), a negative output node (V_(outn)), anda control input (V_(ctrl)) node. In some examples, the components orarrangement of components of a VCDL vary from what is represented forthe VCDL 1100.

FIG. 12 is a schematic diagram of a current-mode logic (CML) XNOR gate1200 (e.g., the autocorrelators in FIGS. 4 and 8-10) in accordance withsome examples. As shown, the XNOR gate 1200 includes a positive supplyvoltage (V_(DD)), a ground node, an A node, an A input node, a B node, aB node, a Y node, and a Y node. The CML XNOR gate 1200 also includesresistors (R7 and R8) and various transistors (Q1-Q6) in the arrangementshown. In some examples, the components or arrangement of components foran autocorrelator vary from what is represented for the CML XNOR gate1200.

FIG. 13 is a schematic diagram of a transconductance (G_(m)) stage 1300(e.g., the transconductance stages in FIGS. 4, 8, and 9) in accordancewith some examples. As shown, the transconductance stage 1300 includes apositive supply voltage (V_(DD)), a ground node, a positive input node(V_(inp)), a negative input node (V_(inn)), and an output current(I_(out)) node. The transconductance stage 1300 also various transistors(M1-M4) in the arrangement shown. In some examples, the components orarrangement of components for a transconductance stage vary from what isrepresented for the transconductance stage 1300.

In the proposed delay adaption loop topologies, an autocorrelationfunction is used. The autocorrelation function of a random data streamdecreases monotonically as the time shift is increased from zero, andgoes to zero when the time shift reaches and exceeds 1 UI. In someexamples, XNOR (or XOR) gates with the input data and its delayedversion as inputs are used to generate this autocorrelation function,and a control loop around this XNOR gate drives the delay towards 1 UIwhile driving the autocorrelation function to zero. In the presence ofISI, the autocorrelation function may have a non-zero tail that exceedstime shift of 1 UI. So instead of locking to 1 UI, it is more robust tolock to 0.5 UI by forcing the autocorrelation function towards 0.5(instead of 0). These 0.5 UI delay elements could then be cascaded togenerate the 1 UI delay. With the proposed delay adaption looptopologies, calibrated delays are created for random data without theneed of a clock in the system (and maintains the delay despite voltageand temperature variations). The proposed delay adaptation looptopologies enables the design of adaptive delay lines for clocklessDFEs, clockless FFEs, loss of lock detectors, and other suchapplications across data rates. With the proposed delay adaptation looptopologies no clock is used, and the solution can be generalized tocreate fractional delays. Also, the solution is scalable across datarates, such that automatic adaptation to the desired delay is possibledespite voltage and temperature variations.

In some examples, an apparatus (e.g., an integrated circuit, chip, orcircuit corresponding to a linear repeater or redriver with a clocklessDFE, a clockless FFE, a loss of lock detector, or other such circuitthat adapts to different data rates) includes a clockless delayadaptation loop configured to adapt to random data. The apparatusincludes a circuit (e.g., a clockless DFE, a clockless FFE, a loss oflock detector, or other such circuit that adapts to different datarates) coupled to the clockless delay adaptation loop. The clocklessdelay adaptation loop includes a cascaded delay line (e.g., the cascadeddelay line 205 of FIG. 2, or VCDLs in FIGS. 4 and 8-10) and anautocorrelation control circuit (e.g., the autocorrelation controlcircuit 206 in FIG. 2) coupled to the cascaded delay line, wherein anoutput of the autocorrelation control circuit is used to generate acontrol signal (e.g., V_(ctrl), V_(ctrl1), V_(ctrl2)) for the cascadeddelay line.

In some examples, the cascaded delay line comprises a VCDL (e.g., theVCDLs in FIGS. 4 and 8-10) and the autocorrelation control circuitcomprises a XNOR gate (e.g., the XNOR gates in FIGS. 4 and 8-10). Insome examples, the autocorrelation control circuit also includes atransconductance stage (e.g., the transconductance stages in FIGS. 4, 8,and 9) coupled to an output of the autocorrelation control circuit. Insome examples, the autocorrelation control circuit also includes acapacitor (e.g., the capacitors in FIGS. 4, 8, and 9) with a first platecoupled to an output of the transconductance stage and with a secondplate coupled to a ground node, wherein the output of thetransconductance stage is the control signal, and wherein the controlsignal locks the clockless delay adaptation loop to a delay value of 1UI. In some examples, the autocorrelation control circuit also includesa digital state machine (e.g., the digital state machine 1008 of FIG.10) coupled to an output of the autocorrelation control circuit. In someexamples, the autocorrelation control circuit includes a DAC (e.g., theDAC 1014 in FIG. 10) coupled to an output of the digital state machine,where an output of the DAC is the control signal, and where the controlsignal locks the clockless delay adaptation loop to a delay value of 1UI.

In some examples, the cascaded delay line comprises a first delay line(e.g., VCDL 402 in FIG. 4) and a second delay line (e.g., VCDL 404 inFIG. 4) in series, wherein each of the first and second delay linescomprises a control input node, and wherein the output of theautocorrelation control circuit is used to generate a control signal(e.g., V_(ctrl) in FIG. 4) input to the respective control input nodesof the first and second delay lines.

In some examples, the cascaded delay line comprises a first delay line(e.g., VCDL 802 in FIG. 8) and a second delay line (e.g., VCDL 804 inFIG. 8) in series, wherein each of the first and second delay linescomprises a control input node, wherein the autocorrelation controlcircuit comprises a first autocorrelator (e.g., XNOR gate 806 in FIG. 8)associated with the first delay line, and wherein an output of the firstautocorrelator is used to generate a control signal (e.g., V_(ctrl1) inFIG. 8) input to the control input node of the first delay line. In someexamples, the autocorrelation control circuit also includes a secondautocorrelator (e.g., XNOR gate 816 in FIG. 8) associated with thesecond delay line, wherein an output of the second autocorrelator isused to generate a control signal (e.g., V_(ctrl2) in FIG. 8) input tothe control input node of the second delay line.

In some examples, the cascaded delay line comprises a first delay line(e.g., VCDL 902 in FIG. 9) and a second delay line (e.g., VCDL 904 inFIG. 9) in series, wherein each of the first and second delay linescomprises a control input node, wherein the autocorrelation controlcircuit comprises a first autocorrelator (e.g., XNOR gate 906 in FIG. 9)associated with the first delay line. In some examples, theautocorrelation control circuit also includes a second autocorrelator(e.g., XNOR gate 916 in FIG. 9) associated with the second delay line,wherein an average output of the first and second autocorrelators isused to generate a control signal (e.g., V_(ctrl) in FIG. 9) input tothe control input nodes of the first and second delay lines.

In some examples, a clockless DFE (e.g., the clock DFE 200 in FIG. 2)includes a summation circuit (e.g., the summation circuit 208 in FIG. 2)configured to provide an output signal (e.g., V_(out) in FIG. 2) bycombining an input signal (e.g., Vin in FIG. 2) and a feedback signal(e.g., the feedback signal 216 in FIG. 2). The clockless DFE alsoincludes a DFE feedback path (e.g., the DFE feedback path 203 in FIG. 2)configured to receive the output signal and provide the feedback signal,wherein the DFE feedback path includes a delay adaptation loop (e.g.,any of the delay adaptation loops of FIGS. 2, 4, and 8-10) configured toadapt to random data. The delay adaption loop includes a VCDL (e.g., theVCDLs in FIGS. 4 and 8-10) with a data input node, a data output node,and a control input node. The delay adaptation loop also includes anautocorrelator (e.g., the XNOR gates in FIGS. 4 and 8-10) coupled to thedata input node and the data output node, wherein an output of theautocorrelator is used to generate a control signal (e.g., V_(ctrl),V_(ctrl1), V_(ctrl2)) input to the control input node. The delayadaptation loop also includes a DFE tap circuit (see e.g., FIG. 1) inseries with the VCDL.

In some examples, the autocorrelator is a XNOR gate, and the delayadaptation loop also includes a transconductance stage (e.g., thetransconductance stages in FIGS. 4, 8, and 9) coupled to an output ofthe autocorrelator. In some examples, the delay adaptation loop includesa capacitor (e.g., the capacitors in FIGS. 4, 8, and 9) with a firstplate coupled to an output of the transconductance stage and with asecond plate coupled to a ground node, wherein the output of thetransconductance stage is the control signal, and wherein the controlsignal locks the delay line to a delay value of 1 UI. In some examples,the clockless delay adaptation loop includes a digital state machine(e.g., the digital state machine 1008 of FIG. 10) coupled to an outputof the autocorrelator. In some examples, the clockless delay adaptationloop includes a DAC (e.g., the DAC 1014 in FIG. 10) coupled to an outputof the digital state machine, where an output of the DAC is the controlsignal, and where the control signal locks the delay line to a delayvalue of 1 UI.

In some examples, the delay adaptation loop comprises a single VCDLcorresponding to the VCDL. In other examples, the VCDL is a first VCDL(e.g., VCDL 802 in FIG. 8) and wherein the delay adaptation loopcomprises a second VCDL (e.g., VCDL 804 in FIG. 8) in series with thefirst VCDL, wherein each of the first and second VCDLs have a controlinput node, wherein the autocorrelator is a first autocorrelator (e.g.,XNOR gate 806 in FIG. 8) associated with the first VCDL, and wherein anoutput of the first autocorrelator is used to generate a control signalinput (e.g., Vctrl1 in FIG. 8) to the control input node of the firstVCDL. In such examples, the delay adaptation loop also includes a secondautocorrelator (e.g., XNOR gate 816 in FIG. 8) associated with thesecond VCDL, wherein an output of the second autocorrelator is used togenerate a control signal input (e.g., V_(ctrl2) in FIG. 8) to thecontrol input node of the second VCDL.

In some examples, the VCDL is a first VCDL (e.g., VCDL 902 in FIG. 9)and wherein the delay adaptation loop comprises a second VCDL (e.g.,VCDL 904 in FIG. 9) in series with the first VCDL, wherein each of thefirst and second VCDLs have a control input node, wherein theautocorrelator is a first autocorrelator (e.g., XNOR gate 906 in FIG. 9)associated with the first VCDL. In such examples, the clockless delayadaptation loop also includes a second autocorrelator (e.g., XNOR gate916 in FIG. 9) associated with the second VCDL, wherein an averageoutput of the first and second autocorrelators is used to generate acontrol signal (e.g., Vctrl in FIG. 9) input to the control input nodesof the first and second VCDLs.

In this description, the term “couple” or “couples” means either anindirect or direct wired or wireless connection. Thus, if a first devicecouples to a second device, that connection may be through a directconnection or through an indirect connection via other devices andconnections. The recitation “based on” means “based at least in parton.” Therefore, if X is based on Y, X may be a function of Y and anynumber of other factors.

Modifications are possible in the described embodiments, and otherembodiments are possible, within the scope of the claims.

What is claimed is:
 1. An apparatus that comprises: a delay adaptationloop configured to adapt to random data; and a circuit coupled to thedelay adaptation loop, wherein the delay adaptation loop comprises: acascaded delay line; and an autocorrelation control circuit coupled tothe cascaded delay line, wherein an output of the autocorrelationcontrol circuit is used to generate a control signal for the cascadeddelay line; wherein the cascaded delay line comprises avoltage-controlled delay line (VCDL), wherein the autocorrelationcontrol circuit comprises: an XNOR gate; a transconductance stagecoupled to an output of the XNOR gate; and a capacitor with a firstplate coupled to an output of the transconductance stage and with asecond plate coupled to a ground node, wherein the output of thetransconductance stage is the control signal, and wherein the controlsignal locks the delay adaptation loop to a delay value of 1 unitinterval (UI).
 2. An apparatus that comprises: a delay adaptation loopconfigured to adapt to random data; and a circuit coupled to the delayadaptation loop, wherein the delay adaptation loop comprises: a cascadeddelay line; and an autocorrelation control circuit coupled to thecascaded delay line, wherein an output of the autocorrelation controlcircuit is used to generate a control signal for the cascaded delayline; wherein the cascaded delay line comprises a voltage-controlleddelay line (VCDL), and wherein the autocorrelation control circuitcomprises: an XNOR gate; a digital state machine coupled to an output ofthe XNOR gate; and a digital-to-analog converter (DAC) coupled to anoutput of the digital state machine, wherein an output of the DAC is thecontrol signal, and wherein the control signal locks the delayadaptation loop to a delay value of 1 unit interval (UI).
 3. Anapparatus that comprises: a delay adaptation loop configured to adapt torandom data; and a circuit coupled to the delay adaptation loop, whereinthe delay adaptation loop comprises: a cascaded delay line; and anautocorrelation control circuit coupled to the cascaded delay line,wherein an output of the autocorrelation control circuit is used togenerate a control signal for the cascaded delay line; wherein thecircuit comprises a decision-feedback equalizer (DFE); wherein theapparatus is a linear repeater device with a driver coupled to an outputof the DFE.
 4. A decision-feedback equalizer (DFE), comprising: asummation circuit configured to provide an output signal by combining aninput signal and a feedback signal; and a DFE feedback path configuredto receive the output signal and provide the feedback signal, whereinthe DFE feedback path includes a delay adaptation loop configured toadapt to random data, and wherein the delay adaption loop comprises: avoltage-controlled delay line (VCDL) with a data input node, a dataoutput node, and a control input node; an autocorrelator coupled to thedata input node and the data output node, wherein an output of theautocorrelator is used to generate a control signal input to the controlinput node; and a DFE tap circuit in series with the VCDL; wherein theautocorrelator is an XNOR gate, and wherein the delay adaptation loopfurther comprises: a transconductance stage coupled to an output of theautocorrelator; and a capacitor with a first plate coupled to an outputof the transconductance stage and with a second plate coupled to aground node, wherein the output of the transconductance stage is thecontrol signal, and wherein the control signal locks the delay adaptionloop to a delay value of 1 unit interval (UI).
 5. A decision-feedbackequalizer (DFE), comprising: a summation circuit configured to providean output signal by combining an input signal and a feedback signal; anda DFE feedback path configured to receive the output signal and providethe feedback signal, wherein the DFE feedback path includes a delayadaptation loop configured to adapt to random data, and wherein thedelay adaption loop comprises: a voltage-controlled delay line (VCDL)with a data input node, a data output node, and a control input node; anautocorrelator coupled to the data input node and the data output node,wherein an output of the autocorrelator is used to generate a controlsignal input to the control input node; and a DFE tap circuit in serieswith the VCDL; wherein the autocorrelator is an XNOR gate, and whereinthe delay adaptation loop further comprises: a digital state machinecoupled to an output of the autocorrelator; and a digital-to-analogconverter (DAC) coupled to an output of the digital state machine,wherein an output of the DAC is the control signal, and wherein thecontrol signal locks the delay adaption loop to a value of 1 unitinterval (UI).
 6. A delay adaptation loop, comprising: a first delayline; a second delay line; an autocorrelation control circuit coupled tothe first delay line and the second delay line, wherein theautocorrelation control circuit provides a control signal to each of thefirst and second delay lines based on an autocorrelation functionapplied to input data and output data of at least one of the first andsecond delay lines; wherein the autocorrelation control circuitcomprises: an XNOR gate; a transconductance stage coupled to an outputof the XNOR gate; and a capacitor with a first plate coupled to anoutput of the transconductance stage and with a second plate coupled toa ground node, wherein the control signal locks each of the first andsecond delay lines to a delay of 0.5 unit interval (UI) or less.
 7. Adelay adaptation loop, comprising: a first delay line; a second delayline; an autocorrelation control circuit coupled to the first delay lineand the second delay line, wherein the autocorrelation control circuitprovides a control signal to each of the first and second delay linesbased on an autocorrelation function applied to input data and outputdata of at least one of the first and second delay lines; wherein theautocorrelation control circuit comprises: a first XNOR gate configuredto received input data and output data of the first delay line; a firsttransconductance stage coupled to an output of the first XNOR gate; afirst capacitor with a first plate coupled to an output of the firsttransconductance stage and with a second plate coupled to a ground node;a second XNOR gate configured to received input data and output data ofthe second delay line; a second transconductance stage coupled to anoutput of the second XNOR gate; a second capacitor with a first platecoupled to an output of the second transconductance stage and with asecond plate coupled to a ground node.
 8. A delay adaptation loop,comprising: a first delay line; a second delay line; an autocorrelationcontrol circuit coupled to the first delay line and the second delayline, wherein the autocorrelation control circuit provides a controlsignal to each of the first and second delay lines based on anautocorrelation function applied to input data and output data of atleast one of the first and second delay lines; wherein theautocorrelation control circuit comprises: a first XNOR gate configuredto receive input data and output data of the first delay line; a firstresistor with a first end coupled to an output of the first XNOR gateand with a second end coupled to an input node of transconductancestage; a second XNOR gate configured to receive input data and outputdata of the second delay line; a second resistor with a first endcoupled to an output of the second XNOR gate and with a second endcoupled to the input node of the transconductance stage, wherein thetransconductance stage is configured to receive a voltage at the inputnode; a capacitor with a first plate coupled to an output of thetransconductance stage and with a second plate coupled to a ground node.9. A delay adaptation loop, comprising: a first delay line; a seconddelay line; an autocorrelation control circuit coupled to the firstdelay line and the second delay line, wherein the autocorrelationcontrol circuit provides a control signal to each of the first andsecond delay lines based on an autocorrelation function applied to inputdata and output data of at least one of the first and second delaylines; wherein the autocorrelation control circuit comprises: an XNORgate; a digital state machine coupled to an output of the XNOR gate,wherein the digital state machine comprises a counter, an accumulator,and a comparator; and a digital-to-analog converter (DAC) coupled to anoutput of the digital state machine.